1. Field of the Invention
The present invention is directed to a method for operating a nuclear magnetic resonance imaging (tomography) apparatus, and in particular to a method employing the generation of a pulse sequence for fast imaging.
2. Description of the Prior Art
Three methods for fast imaging are known in nuclear magnetic resonance tomography, namely the FLASH method, the FISP method and echo planar method, each having various versions.
For example, U.S. Pat. No. 4,707,658 discloses the FLASH method, wherein gradient echoes with small flip angles of the radio-frequency pulse and repetition times that are significantly shorter than the spin-grid relaxation times of the nuclear spins to be investigated are generated. In this known method, as the flip angles are selected smaller, the shorter the repetition times become, i.e., the faster the pulse sequence becomes. The signal-to-noise ratio also decreases with the diminution of the flip angle.
In an especially fast version of the FLASH method, having extremely short repetition times, referred to as the turbo-FLASH method, the magnetization is inverted before every measurement sequence in order to prevent the T1 contrast from collapsing. Due to the necessary spin inversion, however, one must wait for the establishment of an equilibrium of the spins for every new measurement sequence. Continuous measurement in dynamic equilibrium is therefore not possible.
The FISP method, which is disclosed in detail in U.S. Pat. No. 4,769,603, likewise represents a fast gradient echo method wherein, differing from the FLASH method, the phase coding is reset before every radio-frequency pulse.
The method known as the echo planar method, as disclosed in European Application 0 076 054 is even faster than the FLASH method or FISP method. At the beginning of the pulse sequence, an examination subject is subject to an RF excitation pulse under the influence of a slice selection gradient in a first direction. Nuclear spins are thus excited in a slice of the examination subject. After the excitation, a pulse-coding gradient is activated in a second direction and a read-out gradient is activated in a third direction, the first, second and third directions being perpendicular relative to one another. The read-out gradient is composed of a pre-phasing pulse as well as of sub-pulses of alternating polarity. As a result of this alternating polarity of the read-out gradient, the nuclear spins are dephased and in turn rephased in alternation, so that a sequence of nuclear magnetic resonance signals arises. So many signals are thereby acquired after a single excitation that the entire Fourier k-space is scanned, i.e. that the existing data are adequate for the reconstruction of a complete tomogram.
The nuclear magnetic resonance signals are phase coded, sampled in the time domain, digitized, and the numerical values acquired in this way are entered into a raw data matrix. An image of the examination subject is then reconstructed from this raw data matrix on the basis of a two-dimensional Fourier transformation.
The speed advantage of the EPI method is essentially based on the fact that a plurality of signals that are adequate for the reconstruction of a complete tomogram are acquired after an individual excitation. All signals that ultimately represent gradient echoes must be acquired within the T2* decay. The read-out gradient must therefore be very rapidly bipolarly switched, so that considerable hardware demands are made of a corresponding system.